It has been established in two types of experimental renal disease in rats, the remnant kidney model and nephrotoxic serum nephritis models, that phosphorus restriction prevents renal functional deterioration and histological damage. In addition this protective effect of phosphorus restriction is probably mediated through the suppression of parathyroid hormone. This is based on the finding that parathyroidectomy in animals with nephrotoxic serum nephritis exerted a similar protective effect on renal functional deterioration and histological damage. Besides preventing these changes parathyroidectomy had a major effect on the nephrotic syndrome by decreasing the severity of proteinuria, hypoalbuminemia and hypertriglyceridemia in animals with well established nephrotoxic serum nephritis and azotemia suggesting that parathyroid hormone may have two independent effects, one on the nephrotic syndrome and a second on renal histological damage. These studies are being extended to determine 1) if parathyroid hormone effects the nephrotic syndrome in other immunologic and non-immunologic models, 2) what specific effects parathyroid hormone has on lipid metabolism and the permselectivity of the glomerular basement membrane in the nephrotic syndrome, and 3) what effect parathyroid hormone has on humoral and cellular immune function and the chemical and biochemical constituents of the diseased kidney.